Method for preparing an air breathing electrode

ABSTRACT

A METHOD FOR PREPARING AN AIR BREATHING ELECTRODE WHICH COMPRISES APPLYING A CATALYST COMPOSITION TO A METALLIC GRID MEMBER, APPLYING A FLUOROCARBON POLYMER SHEET MATERIAL CONTAINING A POREFORMING AGENT ONTO THE SIDE OF THE CATALYST COMPOSITION AND THEREAFTER REMOVING THE PORE-FORMER FROM THE FLUOROCARBON POLYMER SHEET MATERIAL TO RENDER IT MICROPOROUS. THE FLUOROCARBON POLYMER SHEET MATERIAL IS APPLIED TO THE CATALYST COMPOSITION BY HOT PRESSING TO FIRMLY ATTACH IT THERETO. THE METALLIC SALT CORE-FORMER IS REMOVED FROM THE FLUOROCARBON POLYMER SHEET MATERIAL AFTER IT IS APPLIED TO THE CATALYST COMPOSITION, AND THE PORE-FORMER MAY BE REMOVED BY CONTACTING THE ELECTRODE WITH A LEACHING SOLVENT. AN ALTERNATIVE PROCEDURE COMPRISES APPLYING A LAYER OF A CATALYST COMPOSITION ONTO A FLUOROCARBON POLYMER SHEET MATERIAL CONTAINING A PORE FORMER, PLACING A METALLIC GRID ONTO THE CATALYST LAYER, HOT PRESSING THE GRID/CATALYST/FLUOROCARBON POLYMER TO FORM A UNITARY STRUCTURE, AND THEN REMOVING THE POREFORMER.

United States Patent O 3,594,236 METHOD FOR PREPARING AN AIR BREATHINGELECTRODE David P. Boden, Yardley, Pa., and Jack C. Sklarchuk, Trenton,N.J., assignors t ESB Incorporated N0 Drawing. Filed Nov. 22, 1967, Ser.No. 684,935 Int. Cl. H01m 27/04 US. Cl. 136120 Claims ABSTRACT OF THEDISCLOSURE A method for preparing an air breathing electrode whichcomprises applying a catalyst composition to a metallic grid member,applying a fluorocarbon polymer sheet material containing a poreformingagent onto one side of the catalyst composition and thereafter removingthe pore-former from the fluorocarbon polymer sheet material to renderit microporous. The fluorocarbon polymer sheet material is applied tothe catalyst composition by hot pressing to firmly attach it thereto.The metallic salt pore-former is removed from the fluorocarbon polymersheet material after it is applied to the catalyst composition, and thepore-former may be removed by contacting the electrode with a leachingsolvent. An alternative procedure comprises applying a layer of acatalyst composition onto a fluorocarbon polymer sheet materialcontaining a pore former, placing a metallic grid onto the catalystlayer, hot pressing the grid/catalyst/fluorocarbon polymer to form aunitary structure, and then removing the poreformer.

BACKGROUND OF THE INVENTION The present state of fuel cell developmentrequires the use of very expensive catalyst materials for both oxygenand fuel electrodes and also requires complicated and expensiveauxiliary equipment for eflieient operation. In order to avoid thesedisadvantages of fuel cell power systems, other power sources such asair depolarized cells have been investigated. An air depolarized cellhaving an air or oxygen electrode capable of breathing air or oxygenfrom the atmosphere Would be a substantial improvement in this type ofpower system, for it would eliminate the need for oxygen tanks and otherauxiliary equipment.

During the development of an air breathing electrode, great diflicultywas encountered in preparing an electrode capable of breathing air andalso capable of containing electrolyte within the air depolarized cell.A thin, hydrophobic fluorocarbon polymer sheet material having uniformmicroporosity has been developed, and after applying this fluorocarbonpolymer sheet material to one side of the air electrode, it is capableof breathing air and also retaining electrolyte within the airdepolarized cell. One of the principal problems encountered during thisdevelopment, was how to firmly attach the microporous fluorocarbonpolymer sheet material to the electrode catalyst composition withoutdestroying its uniform microporosity. This problem has been overcome bypreparing the air breathing electrode in accordance with the method ofthis invention.

SUMMARY OF THE INVENTION This invention comprises applying a thin,microporous fluorocarbon polymer sheet material to one surface of an airelectrode in such a manner as to firmly attach it thereto withoutdestroying or otherwise adversely affecting the uniform microporosity ofthe fluorocarbon polymer sheet material. This is accomplished by leavingthe pore-forming agent in the fluorocarbon polymer sheet material duringits application to the catalyst composition and thereafter, removing thepore-forming agent with a leaching solvent.

3,594,236 Patented July 20, 1971 "ice In order to firmly attach thefluorocarbon polymer sheet material to the electrode, it is preferred tohot press the fluorocarbon polymer sheet material onto the catalystcomposition. This method is particularly valuable because it provides anair electrode having uniformly microporous fluorocarbon polymer sheetmaterial firmly applied to one side of the electrode.

DETAILED DESCRIPTION This invention relates to the method for preparingan air breathing electrode which requires applying a thin, microporousfluorocarbon polymer sheet material to an electrode catalyst compositionin such a manner that the uniform microporosity is preserved. In thepreparation of the air electrode, the catalyst composition is generallyprepared first. The catalyst composition generally comprises anelectrically conductive, particulate carrier material, which functionsas a carrier for an electrochemically active catalyst. Carbon is thepreferred carrier material because of its low cost, but other carriermaterials such as finely divided metal powders may be used. Theelectrochemically active catalyst which is applied to the carriermaterial may be selected from the well-known fuel cell catalystmaterials such as silver, gold and metals of the platinum group(platinum, palladium, rhodium, ruthenium, osmium and iridium). It shouldbe noted that particulate carbon material having this type of catalystdeposited thereon is commercially available. It is also possible to usefinely divided catalyst materials without a carrier, but since thecatalysts are generally very expensive, it is preferred to use a lowercost carrier material such as carbon. Furthermore, finely divided carboncan function as the catalyst material without the expensive catalystsmentioned above, but these catalysts substantially improve theefliciency of the electrochemical reaction occurring at the airelectrode, and therefore, it is preferred that one or more of thesecatalysts be present in the catalyst composition.

The catalyst material may be present in amounts ranging from about 0.01to about 10% by weight of the carrier material, with about 5% platinumand 1% silver each having been found satisfactory. It is generallypreferred to use silver as the catalyst for it provides substantiallythe same performance as platinum at a considerable cost saving, and thesilver catalyst usually has a longer life. In addition, the catalystcomposition may also contain a hydrophobic material which functions as awet-proofing agent. The purpose of the wet-proofing agent is to preventelectrolyte from completely covering the surface of the air electrodecatalyst when it is immersed in the aqueous electrolyte. Examples ofwet-proofing agents which may be used are fluorocarbon polymers,silicone resins, or paraflin wax. The wet-proofing agent generallycomprises from about 5 to about 50% by weight of the total catalystcomposition, with about 20% being preferred.

The thin, microporous fluorocarbon polymer sheet material which isapplied to one surface of the air electrode is an important element ofthis invention. This fluorocarbon polymer sheet material must haveuniform microporosity enabling the electrode to breathe air, and themicropores must be of such a small size that the hydrophobic property ofthe fluorocarbon polymer will prevent electrolyte leakage. It has beenfound that the poreforming particles, e.g. a metallic salt, should besufficiently small to pass through a ZOO-mesh screen, equivalent to aparticle size of about 73 microns or less, and particles passing througha 325-mesh screen, equivalent to a particle size of about 40 microns orless, have been found to yield satisfactory microporouspolytetrafluoroethylene sheet material. In addition, .the fluorocarbonpolymer sheet material must be firmly attached to the catalystcomposition in order to permit a prolonged operation of the airdepolarized cell in which the air electrode is employed. Thefluorocarbon polymer sheet material may be prepared from variousfluorocarbon polymers such as polytetrafluoroethylene,polytrifluoroethylene, polyvinyl fluoride, polytrifluorochloroethyleneand copolymers thereof, with polytetrafluoroethylene being particularlypreferred. In general, fluorocarbon polymer sheet material having athickness ranging from about 5 to about 30 mils has been foundsatisfactory.

In preparing an air electrode, the catalyst composition is generallyprepared first. This may be accomplished by forming a slurry of thecatalyst-containing carrier particles in water. The slurry is stirredrapidly and a diluted solution of the wet-proofing agent, preferablypolytetrafluoroethylene, is slowly added to the slurry. After ahomogeneous composition is formed. it is washed thoroughly in waterand/or an organic solvent such as acetone and then it is allowed to dry.The electrode is formed by pressing the catalyst composition onto aclean porous metallic grid member using pressure ranging from about5,000 to about 30.000 p.s.i.

It should be noted that the metallic grid member should not corrode inthe electrolyte in which the electrode will be immersed. For alkaline orneutral electrolytes, it is generally preferred to use nickel screen asthe grid member. In acid electrolyte, tantalum or columbium are thepreferred grid member metals.

Another requirement of this invention is that the fluorocarbon polymersheet material be capable of being rendered microporous after it isapplied to one surface of the catalyst composition. or the catalystcomposition is applied to its surface. The fluorocarbon polymer sheetmaterial is prepared by forming a mixture of a fluorocarbon polymer(polytetrafluoroethylene is preferred), metallic salt particles whichfunction as a pore-forming agent (sodium carbonate or calcium formateare preferred), and a paraflin wax which functions as a binder and alubricant. This mixture is mixed in a high speed blender and thereafteris formed into sheet material using conventional means such as a rubbermill. After the mixture is formed into a sheet. the paraflin wax isremoved by treating the sheet with an organic solvent such as byimmersing it in an acetone bath. Then the sheets are sintered in asintering furnace at the appropriate temperature for sintering thefluorocarbon polymer. After the sheet is sintered and while it stillcontains the metallic salt pore-forming particles. it is ready forapplication to the catalyst composition of the air electrode.

The critical part of the preparation of the air electrode involves theapplication of the fluorocarbon polymer sheet material onto the catalystcomposition. The catalyst composition may be pressed onto the metallicgrid member prior to applying the fluorocarbon polymer sheet.Alternatively, the catalyst composition and the fluorocarbon polymersheet can be pressed onto the grid in one step by applying a layer ofcatalyst onto the fluorocarbon polymer sheet, placing a grid on thecatalyst layer, and then hot pressing these components to form a unitarystructure. In carrying out this operation, several problems wereencountered. If the fluorocarbon polymer sheet material has already beenrendered microporous, only very slight pressure can be used to apply itto the air electrode and this causes poor adherence to the electrode. Inaddition. even slight pressure causes destruction of some of themicropores. Another method which was tried involved spraying afluorocarbon polymer emulsion onto one surface of the air electrode, butthis also resulted in poor adherence and also non-uniform porosity. Athird method which proved to be more satisfactory involved pressing themicroporous fluorocarbon polymer sheet material on the air electrode andplacing a plastic mesh material lVEXAR) over the fluorocarbon polymersheet. Pressure was applied to the plastic mesh which adhered thefluorocarbon polymer under the strands of the mesh to the electrode, butwhich prevented the microporous fluorocarbon polymer sheet under theopenings of the plastic mesh from becoming compressed and it retainedits porosity. Whereas this method was an improvement, it had severaldisadvantages. Some of the microporous fluorocarbon sheet ma.- terialhas its porosity destroyed and therefore blocks off a portion of the airelectrode. Another problem which was encountered involved the plasticmesh material penetrating the fluorocarbon polymer sheet material whichcaused the air electrode to leak electrolyte. Furthermore, this methoddid not provide suflicient adherence of the fluorocarbon polymer sheetmaterial to the catalyst composition as required in air-metal cellsoperated through many discharge-charge cycles.

In accordance with the method of this invention, these problems havebeen overcome by applying the fluorocarbon polymer sheet material to thecatalyst composition while it still retains the pore forming agent. Inaccordance with this method, the fluorocarbon polymer sheet materialcontaining the pore former is hot pressed onto the catalyst compositionat a temperature ranging from about 200 F. to about 400 F. and at apressure ranging from about 5,000 p.s.i. to about 30,000 p.s.i. Thisheat and pressure is maintained for about 2 minutes. I-Iot pressingtemperatures below 200 F. result in poor adhesion of the polymer sheetto the catalyst composition. This hot pressing operation firmly adheresthe fluorocarbon polymer sheet material to the catalyst composition, andthereafter. the electrode containing the fluorocarbon polymer sheetmaterial is immersed in water or other suitable solvent to leach out thepore former from the fluorocarbon polymer. Depending on the solubilityof the pore former and the thickness of the fluorocarbon polymer sheetmaterial, removal of the pore former may require immersion in theleaching solvent for about /2 to about 24 hours. It is preferred to usefresh leaching solvent periodically to ensure substantially completeremoval of the pore former.

Air electrodes prepared by this method have given excellent performanceand have been substantially leakproof. These electrodes are particularlyuseful in air depolarized cells where the side of the air electrodecoated with the microporous fluorocarbon polymer can serve as the sidewall of the cell. It is particularly preferred to use two of these airbreathing electrodes per cell in which case each air electrode serves asthe side wall of the cell. In this manner the atmospheric air has accessto the air electrode through the microporous fluorocarbon polymer sheetmaterial which is also capable of retaining the aqueous electrolytewithin the cell. Whereas these air breathing electrodes are particularlyuseful in air depolarized cells, they may also be used in otherapplicatrons such as fuel cells.

The following examples illustrate the preparation of an air breathingelectrode in accordance with this inventlon.

EXAMPLE I Calcium formate (Ca(CHO was dried at 248 F. and sieved througha 325 mesh screen. A 5/1 mix was prepared by mixing 250 g. of the Ca(CHOpowder with 83.3 g. of a polytetrafluoroethylene emulsion (Tefloncontaining g. of polytetrafluoroethylene polymer. The emulsion wasdiluted with 50 cc. water, stirred rapidly, and then the Ca(CHO powderwas added slowly. After thoroughly mixing this composition for 10minutes, it was dried at 257 F. to complete dryness and then reduced tomicroparticles in a high speed blender. 15 g. of paraflin wax wasthoroughly mixed into this composition prior to forming it into sheetmaterial.

A rubber mill was used to form sheets from the blended mix. The backroll of the mill was heated to F. and the front roll to F. The spacingof the rolls was adjusted to yield a sheet 20 mils thick. The powderedmix was poured onto the rubber mill, rolled once, stripped ofl? of themill roll, folded over and rolled again. This rolling-stripping-rollingprocedure was repeated to ensure correct thickness and uniformity, afterwhich the material was stripped from the rolls and retained as sheetmaterial. The sheets were allowed to cool and became still but durableand easily handled.

The rolled sheets were then placed in a Soxhlet Extractor containingwarm acetone (just below boiling point) to remove the parafiin wax. Thisextraction process required about /2 hour. The excess acetone carried bythe sheet material was blotted off, and the sheet was placed in a coldsintering furnace. The furnace was heated slowly to 650 F. and thepolytetrafluoroethylene was sintered at 650 F. for about /2 hour, andthereafter, it was permitted to cool to room temperature.

A catalyst composition was prepared by forming an aqueous slurry of 5%platinum catalyst deposited on a carbon carrier, which is commerciallyavailable (Englehard). This slurry was stirred rapidly and a dilutedsolution of a polytetrafluoroethylene emulsion (Teflon 30) was slowlyadded to the slurry in an amount to provide 20% by weight ofpolytetrafluoroethylene per weight of the total dry composition. Aftermixing the polytetrafluoroethylene into the catalyst composition to forma homogeneous mixture, the composition was Washed thoroughly in water,then in acetone and again in water and dried. 3 g. of this wet-proofedcatalyst composition was pressed into a clean 75 mesh nickel screen at14,000 p.s.i. and at room temperature, forming an air electrode havingan area on each side of 2.5 inches by 2.5 inches.

Onto one side of the air electrode, a piece of the sinteredpolytetrafluoroethylene sheet material, still containing the calciumformate pore-former, was pressed at 14,000 p.s.i. and 400 F. for 2minutes. After hot pressing the polytetrafluoroethylene sheet onto theair electrode, the electrode containing the sheet was immersed in warmwater (192 F.) to remove the calcium formate poreformer. To ensurecomplete removal of the pore-former, the electrode was allowed to remainin the warm water for about 16 hours (overnight), and thereafter it wasremoved from the water and permitted to dry.

This electrode was tested as a half-cell in 27% potassium hydroxideelectrolyte and yielded the following voltages under various loads:

VOLTS VS. H

Open circuit 0.980 Ma./cm.

EXAMPLE 11 Air electrodes were prepared in accordance with the methodset forth in Example I with the exception that the temperature at whichthe polyterafluoroethylene was hot pressed onto the catalyst compositionwas varied. Temperatures of 200 F., 300 F., 400 F. and 500 F. wereselected and the performance of these air electrodes in half-cells using27% potassium hydroxide electrolyte was determined. The results were asfollows:

The electrodes hot pressed at 500 F. could not be tested for during thehot pressing step the catalyst composition ignited causing thepolytetrafluoroethylene to flow and warp, and the burned catalystcomposition fell away from the nickel screen and thepolytetrafluoroethylene sheet. It was concluded that hot pressingtemperatures greater than 200 F. and less than 500 F. are satisfactoryfor the method of this invention when using polytetrafluoroethylene. Ofcourse, it may be possible to vary the hot pressing temperature it otherfluorocarbon polymer materials or catalyst compositions are used.

Having completely described this invention, what is claimed is:

1. A method for preparing an air breathing electrode which comprisesapplying a catalyst composition to a metallic grid member to form anelectrode, hot pressing a fluorocarbon polymer sheet containing apore-forming agent onto one side of said catalyst composition, and thencontacting the electrode with a leaching solvent for the pore-former toremove it from the fluorocarbon polymer sheet and thereby render itmicroporous.

2. A method in accordance with claim 1 in which the pore-forming agenthas a particle size less than about 73 microns.

3. A method in accordance with claim 2 in which the fluorocarbon polymeris polytetrafluoroethylene and the hot pressing operation is performedat a temperature greater than about 200 F. and less than about 500 F.

4. A method in accordance with claim 2 in which the catalyst compositioncomprises an electrochemically active catalyst deposited on anelectrically conductive, particulate carrier material and a hydrophobicwet-proofing agent.

5. A method in accordance with claim 4 in which the fluorocarbon polymeris polytetrafluoroethylene and the pore-forming agent has a particlesize less than about 40 microns.

6. A method for preparing an air breathing electrode which comprisesapplying a thin layer of a catalyst composition onto a fluorocarbonpolymer sheet which contains a pore-forming agent, placing a metallicgrid on the catalyst composition layer, hot pressing the grid/ catalystcomposition/fluorocarbon polymer sheet to form a unitary electrodestructure, and then contacting the electrode structure with a leachingsolvent for the pore former to remove it from the fluorocarbon polymersheet and thereby render it microporous.

7. A method in accordance with claim 6 in which the hot pressingoperation is performed at a temperature between about 200 F. and about500 F.

8. A method in accordance with claim 6 in which the pore-forming agenthas a particle size less than about 73 microns.

9. A method in accordance with claim 6 in which the catalyst compositioncomprises an electrochemically active catalyst deposited on anelectrically conductive, particulate carrier material.

10. A method in accordance with claim 7 in which the fluorocarbonpolymer is polytetrafluoroethylene.

References Cited UNITED STATES PATENTS 3,281,511 10/1966 Goldsmith264127 3,385,736 5/1968 Dei-bert 136120 3,385,780 5/1968 I-Ming Feng136-120X 3,432,355 3/1969 Niedrach et al. 136-86 WINSTON A. DOUGLAS,Primary Examiner M. J. ANDREWS, Assistant Examiner

